A command line tool to interact with the PostgreSQL Extension Network

For example, to install the semver extension, the client can be invoked as:

$ pgxn install semver

which would download and compile the extension for one of the PostgreSQL
servers hosted on the machine and:

$ pgxn load -d somedb semver

which would load the extension in one of the databases of the server.

The client interacts with the PGXN web service and a Makefile provided by
the extension. The best results are achieved with makefiles using the
PostgreSQL Extension Building Infrastructure; however the client tries to
degrade gracefully in presence of any package hosted on PGXN.